Apparatus for depositing low stress films

ABSTRACT

X-ray masks are typically made by depositing and patterning a layer of heavy metal on a thin supporting membrane. The metal layer must have a relatively low stress to prevent stress-induced deformation of the pattern. Tungsten films having excellent stress characteristics are produced by employing a continuously operating capacitance-based measurement technique to allow adjustment of the deposition conditions in rapid response to changes in stress of the film being deposited.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 08/166,672 filed Dec. 14,1993, now U.S. Pat. No. 5,382,340, which is a continuation of Ser. No.07/850,639 filed Mar. 13, 1992 now abandoned.

This application has been filed concurrently with U.S. patentapplication Ser. No. 07/850,637, pending which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to fabrication of x-ray masks and, in particular,masks including a patterned metal on a membrane.

2. Art Background

As design rules in the manufacture of devices, e.g. integrated circuitsopto-electronic devices, and micro-mechanical structures, becomesmaller, the radiation employed for lithography, in turn, must be of acorrespondingly shorter wavelength. Thus, for example, when the designrule is below 0.5μ, use of short wavelength radiation such as x-rayradiation (radiation having a wavelength typically in the range 4 to 150Å) has been suggested.

During exposure, energy incident on a mask which defines a pattern istransmitted in this pattern to expose an underlying energy sensitivematerial. The energy sensitive material after this exposure isdelineated into the pattern by development and employed in themanufacture of the desired device. For x-ray exposure, the mask isgenerally a membrane stretched across a supporting structure, forexample, a ring with a region patterned in a metal coating the membranesurface. Typically, the membrane is a material such as Si, SiN_(x) (x istypically between 1 and 1.3) or SiC, and has a thickness generally inthe range 0.1 to 4 μm.

Since the membranes must be quite thin to avoid excessive attenuation ofincident energy, substantial stress, i.e. stress greater than 50 MPa,imposed on the membrane from the overlying metal pattern is unacceptablebecause it causes unacceptable distortion of the pattern. Therequirement of limited stress, in turn, imposes substantial limitationson the process of forming the overlying metal pattern.

In a typical mask fabrication procedure, a layer of metal is depositedon a membrane such as by sputtering. A pattern in polymeric material isformed over the metal layer, and the metal regions not covered by thepolymeric material are removed by etching. Subsequent removal of theoverlying polymeric material leaves a patterned metal overlying themembrane.

Various materials have been suggested for use in the metal layer.Although gold is relatively easy to deposit, its presence in devicemanufacturing environments and in particular, integrated circuitmanufacturing environments, is not preferred. Gold impurities, even inextremely small mounts, introduced into an integrated circuit oftensubstantially degrade the properties and reliability of the device.Stress in gold films is also known to change with time, even at roomtemperature. Recent studies indicate that at temperatures above 70° C.,stresses increase rapidly. Therefore, materials other than gold havebeen investigated.

One alternative to gold is tungsten. Although tungsten is consideredcompatible with an integrated circuit manufacturing environment,tungsten films deposited on a membrane generally induce substantialcompressive or tensile stress that ultimately distorts the requiredpattern or even produces membrane failure. Various attempts have beenmade to reduce the stress associated with the deposition of tungsten.For example, as described by Y. C. Ku et al, Journal of Vacuum Science &Technology, B9, 3297 (1991), a monitoring method is employed fordetermining stress in the tungsten being deposited. This monitoringmethod is based on the resonant frequency f of a circular diaphragm ofthe composite structure which, in turn, is related to the stress by theequation: ##EQU1## where r is the radius of the membrane, σ_(m), ρ_(m),and t_(m) are stress, density, and thickness of the membranerespectively, and the corresponding terms such as σ_(f) are stress,density, and thickness respectively, of the film. Since the density ofthe film and membrane are generally known, this equation allowscalculation of stress once the resonant frequency and film thickness aremeasured.

Ku and coworkers, used a commercially available optical distancemeasuring device to monitor diaphragm position. Movement of thediaphragm was induced by electrostatic forces applied to the diaphragmfrom an electronic oscillator-driven capacitively coupled electrode. Theoscillator frequency was slowly swept to allow location of the diaphragmmechanical resonance and from this value, the stress was determined.

SUMMARY OF THE INVENTION

It has been found that frequent and rapid measurement of stress isrequired to allow adjustments in a tungsten deposition system so thatstress in the deposited tungsten is meaningfully reduced. Resonantfrequency determinations allowing such adjustment at least 6 times perminute, are required to significantly improve stress characteristics.This measurement performance is advantageously achieved in a resonantfrequency technique by employing a single electrode monitoring system.This system capacitively drives the diaphragm and simultaneouslydetermines its frequency by maintaining it in mechanical oscillation atits resonant frequency. Use of a single multi-function electrode in thisway is quite difficult since the system must exhibit negligiblecrosstalk between the measuring and driving functions. The system mustalso be immune to the very high power rf frequency typically employed inthe sputtering deposition procedure itself. Further, the capacitancefrom the backplate to the diaphragm is quite small i.e. less than 20 pF,while associated parallel stray capacitances to ground are generallysignificantly larger, compounding the difficulties.

These problems are overcome, and rapid measurement is achieved,utilizing the apparatus shown schematically in FIG. 1. A bifilartransformer (1), high voltage emitter follower 2, and shielded cable 3are employed both to avoid these various problems and to maintain thediaphragm in continuous oscillation. Additionally, since the diaphragmis maintained at its resonant frequency, even though the film thicknessis continuously changing, continuous determinations of stress areavailable. In this manner stress is reducible by adjusting processparameters, such as sputtering gas pressure and/or rf power, in responseto the stress measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus for measuringstress in a growing tungsten film.

FIG. 2 is a plot of resonant frequency vs. sputter time for variouspressure levels during sputtering of a tungsten film.

FIG. 3 is a plot of membrane deflection at the edge of a tungsten film.

DETAILED DESCRIPTION

As discussed, the invention involves the realization that to controlstress during deposition of metals on a membrane, it is necessary tofrequently measure this stress during deposition and adjust accordingly.Typically, for membranes having thicknesses in the range 0.1 to 4 μmformed of materials such as Si and SiN_(x) and with deposits of metalssuch as tungsten, at least 6 measurements per minute should be made.Stress in the evolving metal film is then adjusted by correctingparameters such as sputtering gas pressure and/or rf power. The totalstress depends on these parameters in a complicated manner, buttypically compressive stress decreases with an increase in sputteringgas pressure or with a corresponding decrease in rf power.

Although the particular method employed to obtain the necessarymeasurement of stress is not critical, previous techniques (involvingrelatively slow frequency scanning to establish a membrane resonantfrequency) are clearly inadequate. By contrast it has been found that atechnique which maintains the membrane at its resonant frequency whileutilizing a single driving and measuring electrode, is particularlyadvantageous in this regard.

In the one-electrode technique, the capacitance between the membrane 4and the electrode 5 is measured and electronically processed in such away that the output is a linear function of the distance between theelectrode and the membrane. (See G. L. Miller U.S. Pat. No. 4,893,071,dated Jan. 9, 1990, (which is hereby incorporated by reference) andespecially FIGS. 8 and 9 with accompanying text in column 6, line 46, tocolumn 7, line 60.) From this measurement, a voltage is made availablewhich indicates the position of the membrane, i.e. its distance from thebackplate. This voltage is then suitably added to a large, fixed, highvoltage (typically approximately 150 volts) and applied back to theelectrode 5 via the emitter follower 2 and transformer 1. The operationof this whole loop is such as to continuously maintain the diaphragm inmechanical oscillation at its resonant frequency. Measurement of thatfrequency, coupled with the use of Equation 1, allows the stress to bedetermined.

A system for achieving this result is shown in FIG. 1. The diaphragm 4with its metal layer 6 being deposited, is shown relative to anelectrode 5. This electrode is driven by an rf oscillator 7 through abifilar, one-to-one transformer 1. The rf output of the oscillator iscoupled through this transformer to the electrode and is also imposed ona driven shield 3. Since the shield and the lead to the electrode aremaintained at the same RF potential, no error due to capacitance betweenthe shield and the center lead of the cable is introduced.

The capacitive measurement of distance using a feedback loop through anrf rectifier 8, and comparison to a reference input 9, has beendiscussed in U.S. Pat. No. 4,893,071, dated Jan. 9, 1990, which ishereby incorporated by reference (with particular reference to FIGS. 8and 9). Additionally, related distance measurements based on capacitancehave also been discussed in a publication by G. L. Miller in IEEETransactions on Electron Devices, ED-19, pages 1103-1108, October, 1972.

The disc electrode 5 is driven with RF (typically approximately 1 V p--pat 3 MHz) via a toroidal bifilar transformer 1. The far end of thesecondary of this transformer is connected to the emitter of a highvoltage transistor emitter follower 2 (all power supply and biasingarrangements have been omitted for clarity). Essentially all of the RF 3MHz displacement current flowing from the disc 5 to the diaphragm 4therefore flows out of the collector of 2. (Note that the lead to thedisc itself is provided with an accurately driven shield 3 to remove thedead capacitance effect.)

The RF current from the emitter follower 2 collector passes through atuned amplifier (not shown) to a rectifier, the output of which istherefore a measure of the disc to diaphragm spacing. The rectifieroutput is then compared with a constant (demanded) value 9 and the errorsignal between the two is used in turn to servo the oscillator 7(typically 3 MHz) amplitude. In this way the oscillator 7 amplitudeitself is accurately and linearly proportional to the position of thediaphragm, i.e. is a linear measure of the spacing from the diaphragm 4to the backplate 5. This is necessarily so since the operation of thiswhole electronic loop is such, in effect, as to force a constantmagnitude of RF displacement current through the capacitor formed by thebackplate 5 and the membrane 4. The system output voltage is simply alinear measure of the magnitude of the RF voltage needed to achieve thisend. As such it is proportional to the spacing between the diaphragm andthe backplate.

Given such a position signal it is then only necessary to appropriatelyfeed it back as a DC level through emitter follower 2 to cause thediaphragm to constantly oscillate at its resonant frequency. As apedagogic aid it is possible to visualize this process physically. Aslong as the diaphragm is moving towards the backplate the DC voltageacross the gap is increased above its static value of approximately 150volts. While the diaphragm is moving away from the backplate the voltageis correspondingly decreased. The associated electrostatic forces causethe diaphragm to oscillate at its resonant frequency. A separate loopstabilizes the magnitude of the diaphragm oscillatory motion by servoingthe feedback voltage amplitude. This subsidiary loop also providesdamping, or Q, information.

It is desirable to maintain the stress level at a relatively low value,e.g. below 50 MPa, preferably below 10 MPa. Thus, the deposition processparameters as previously discussed are adjusted until the outputindicates an appropriate reduction in stress level.

The following example is illustrative of the techniques involved in theinvention.

EXAMPLE 1

A 1 μm silicon membrane having a stress of approximately 100 MPa wasprepared as described in L. E. Trimble et al., SPIE, Vol. 1263,"Electron Beam, X-Ray, and Ion-Beam Technology: SubmicrometerLithographies IX" (1990), pp. 251-258. This membrane was placed on thesample holder of a conventional sputtering apparatus described inconcurrently filed U.S. patent application Ser. No. 07/850,637, pending.The apparatus was configured such that the gap between the measurementelectrode (approximately 2 cm diameter) and the membrane was 250 μm.(The measuring circuitry was, as shown in FIG. 1.) The chamber wasevacuated to a base pressure of approximately 1×10⁻⁷ Torr. An argon gasflow rate was established to maintain the chamber pressure atapproximately 20 mTorr. (This pressure was chosen to be near thecompressive to tensile transition pressure of 18 mTorr so thatadjustments necessarily performed during deposition would not beexcessively large. The determination of this transition pressure wasdone as described by R. R. Kola et al, in Journal of Vacuum Science andTechnology, B9, page 3301 (1991).)

A plasma was struck in the argon at 13.56 MHz with a power density of1.6 W/cm² to induce sputtering from an 8 inch tungsten target having apurity of 99.999%. After approximately 5 minutes, a shutter positionedbetween the target and the sample was opened. The resonant frequency, asdetermined from the measured voltage from the electronic circuitry andEquation 1, immediately dropped by about 2.5 KHz due to the temperaturedifference between the thin membrane and the thick silicon substrate.The temperature equilibrated in approximately 5 minutes. (This frequencydrop in equilibration is shown in FIG. 2 in the left hand portion of thecurve. Measurements were delayed slightly from initial shutter openingin this Example to allow the membrane to come into tensile stress and,thus, to avoid any possibility of membrane fracture.) The resonantfrequency was then continuously monitored and the pressure adjusted sothat this measured resonant frequency followed, as the depositedthickness increased, the frequency trend predicted by Equation 1 forzero stress.

The predicted resonant frequency in Equation 1, however, does notprovide for temperature effects. To correct for temperature effects, theresonant frequency of the composite membrane for the final desireddeposited thickness was empirically determined under identicaldeposition conditions using a series of control samples. In thesesamples, the deposition procedure described in this example was followedto the final deposited thickness of 0.5 μm. The deposited tungsten wasremoved from half the membrane, and the membrane deflection at theresulting tungsten edge was measured using a WYKO opticalinterferometer. The final resonant frequency of the sample showing zerodeflection (as shown in FIG. 3) for this interferometric measurement isthe temperature corrected, zero stress frequency. The determined zerostress resonant frequency, under the conditions employed, was 3.85 KHz.

Adjustments during deposition were continued so that at the finalthickness, the resonant frequency measured 1.1 KHz. (The resonantfrequency during the run as a result of pressure adjustments to controlstress is shown in FIG. 2.) The shutter was then closed, inducing theresonant frequency to increase by about 2.7 KHz since the membranecooled substantially faster than the substrate. (This increase wascompensated for so that the final room temperature frequency of themembrane was 3.85 KHz.) The sample was then allowed to cool in flowingargon for approximately 10 minutes. The sample was evaluated by removingthe tungsten film from half the wafer. Straight interference fringesacross the resulting tungsten edge indicated a stress very close tozero.

The same procedure was repeated for a silicon nitride membrane on asilicon substrate and a silicon nitride membrane on a glass substrate.In each case, final tungsten stresses below 10 MPa were achieved.

We claim:
 1. An apparatus, comprising:a source that is operable todeposit a film on a substrate; and a system for rapidly measuring theresonant frequency of the substrate having the film growing thereon, thesystem comprising a single electrode and a positioner operable to placethe substrate opposite the electrode, an oscillation controller forimposing an oscillating s signal on the electrode, a capacitive detectorthat measures the distance between the electrode and the substrate in afirst feedback loop, and a second feedback loop responsive to thecapacitive detector and cooperating with the oscillation controller tomaintain the substrate with the film growing thereon at its resonantfrequency.
 2. The apparatus of claim 1 further comprising means foradjusting a deposition process parameter in response to a measurementmade by said system for rapidly measuring the resonant frequency of asubstrate having a film growing thereon.
 3. An r.f. sputteringapparatus, comprising:a source that is operable to deposit a film on asubstrate; and a system for rapidly measuring the resonant frequency ofthe substrate having the film growing thereon, the system comprising asingle electrode and a positioner operable to place the substrateopposite the electrode, an oscillation controller for imposing anoscillating signal on the electrode, a capactive detector that measuresthe distance between the electrode and the substrate in a first feedbackloop, and a second feedback loop responsive to the capacitive detectorand cooperating with the oscillation controller for oscillating theelectrode to maintain the substrate with the film growing thereon at itsresonant frequency.
 4. The r.f. sputtering apparatus of claim 3 furthercomprising means for adjusting a deposition process parameter inresponse to a measurement made by said system for rapidly measuring theresonant frequency of a substrate having a film growing thereon.